This exploratory project is designed to advance an unexpected finding that resulted from our program project grant on improving grant function in parkinsonian non-human primates. As part of the design we tested the ability of "helper" grafts to stimulate and guide the extension of neurites from grafts of ventral mesencephalon that contained dopaminergic neurons. We found that neurites from dopamine neurons could bridge the distance between the brain stem and the striatum and that they could extend from one distant helper graft to a second and ultimately to a third that was located in the striatum. This is an important finding with respect to the potential for reconstruction of elements of the nigrostriatal pathway because it places the grafted dopamine neurons in the mesencephalon where they have the opportunity to be contacted by appropriate afferent inputs. Graft therapy for Parkinsonism in both animal models and humans has utilized the striatum as the graft site for placement of dopaminergic neurons. While this convenient placement resulted in the release of dopamine directly into the region of the striatal target neurons, its shortcoming is that it places the donor neurons in a position that is removed from the normal pathway. In some clinical trials patients who initially showed some level of motor improvement eventually showed dyskinesias that are postulated to be due to uncontrolled or excessive release of dopamine by the grafts. The interesting thing about our finding is that we used striatal "helper" grafts to bridge the relatively long distance between the rostral mesencephalon and caudal striatum in the non-human primate and maintained the correct caudal to rostral position of the dissected pieces of the striatal anlagen. Thus, the dopamine neurites first reached the developing caudal portion of the striatum, and then somewhat remarkably continued to grow in a rostral direction to reach the second, later developing middle portion of the striatal donor graft. Ultimately, the fibers reached the third striatal "step" which was located in the host striatum. We hypothesize that a developmental gradient exists for the striatum from caudal to rostral that normally guides dopamine fibers that approach the striatum from the brain stem, and that a progressive wave of striatal growth factor activity exists to account for the continued extension of these fibers to reach the most distant sites within the striatum. Thus, we propose to test this in rodents prior to any further exploration in non-human primates. The program project grant is not going to be submitted for renewal which is a decision made by the executive committee of the P01 and if this exploratory proposal in rodent is awarded and proves successful then we will utilize this information to plan additional tests in the non-human primate of helper graft circuit reconstruction in behaviorally impaired animals as a possible future R01. PUBLIC HEALTH RELEVANCE: The proposed study has the potential to identify a key aid for successful circuit reconstruction of the dopaminergic pathway that is responsible for the manifestation of the signs and symptoms of Parkinson's disease. Our earlier studies in non-human primates provided a novel finding that can be tested more fully presently in a rodent model prior to moving forward with the more complex primate. We believe that more complete circuit reconstruction is crucial for optimizing graft function.